1. Unit 6 Chemical
Reactions

2. Indicators of chemical reactions
Emission of light or heat
Formation of a gas
Formation of a precipitate
Color change
Emission of odor

3. Review….Count The Atoms
CaCl2
Zn(NO3)2
2NaOH
3H2O
One Calcium/2 Chlorine
1 Zinc/2Nitrogen/6Oxygen
2 Na, 2 O, 2 H
6 H, 3 O

4. Unit 6 Describing Chemical Reactions
OBJECTIVES:
-Describe how to write a word equation.
-Describe how to write a skeleton equation.
- Describe the steps for writing a balanced chemical equation.

5. All chemical reactions:
have two parts
Reactants - the substances you start with
Products- the substances you end up with
The reactants turn into the products.
Reactants ® Products

6. - Page 321
Products
Reactants

7. Anatomy of a Chemical Reaction
FORWARD ARROW: Means produces or yields, etc.
DELTA: Shows that heat is supplied to the reaction
Reactants
Products
CH4 (g) + 2O2 (g) CO2 (g) + 2H 2O (g)
SUBSCRIPT: Tells how many atoms there are of an element
PLUS SIGN: Used to separate each reactant or each product
COEFFICIENT: Tells how many of each reactant or each product there is
ITALICIZED ABBREVIATION: Tells what physical state that reactant or product is in

8. Symbols used in equations
(s) after the formula –solid Cu(s)
(g) after the formula –gas H2 (g)
(l) after the formula -liquid H2O(l)
(aq) after the formula - dissolved in water, an aqueous solution. CaCl2 (aq)

9. Symbols used in equations
indicates a reversible reaction.
shows that heat is supplied to the reaction.
, or is used to indicate a catalyst used supplied, in this case, platinum.
, indicates a pressure other than STP

10. Describing chemical reaction
The way atoms are joined is changed
Atoms are not created or destroyed.
Can be described several ways
1. In a sentence
ex: Copper reacts with chlorine to form copper (II) chloride.
2. In a word equation
ex: Copper + chlorine ® copper (II) chloride
Cu(s) + Cl2(g) ® CuCl2(aq)

11. Formula Equation Uses formulas and symbols to describe a reaction
doesn’t indicate how many.
All chemical equations are sentences that describe reactions.

12. #3. The Skeleton Equation
Uses formulas and symbols to describe a reaction
but doesn’t indicate how many; this means they are NOT balanced
All chemical equations are a description of the reaction.

13. Write a skeleton equation for:
Solid iron (III) sulfide reacts with gaseous hydrogen chloride to form iron (III) chloride and hydrogen sulfide gas.
Nitric acid dissolved in water reacts with solid sodium carbonate to form liquid water and carbon dioxide gas and sodium nitrate dissolved in water.

14. Now, read these equations:
Fe(s) + O2(g) ® Fe2O3(s)
Cu(s) + AgNO3(aq) ® Ag(s) + Cu(NO3)2(aq)
NO2(g) N2(g) + O2(g)

15. Summary of Symbols

16. Don't Forget: Cl I F H BrON
(The Diatomic Elements!!!)

17. Diatomic elements There are 8 elements that never want to be alone.
They form diatomic molecules.
H2 , N2 , O2 , F2 , Cl2 , Br2 , I2 , and At2
The –ogens and the –ines
1 + 7 pattern on the periodic table

19. Convert this to an equation
Solid iron (III) sulfide reacts with gaseous hydrogen chloride to form iron (II) chloride and hydrogen sulfide gas.
Fe2S3 (s) + HCl(g) ® FeCl2 (s) + H2S(g)

20. Convert this to an equation
Nitric acid dissolved in water reacts with solid sodium carbonate to form liquid water and carbon dioxide gas and sodium nitrate dissolved in water.
HNO3 (aq) + Na2CO3 (s) ® NaNO3 (aq) + H2O(l)

21. The other way
Fe(s) + O2(g) ® Fe2O3(s)
Solid iron reacts with oxygen gas to form solid iron oxide (rust).

22. Ag (s) + H2S (g) + O2 (g) ® Ag2S (s) + H2O
A silver spoon tarnishes. The solid silver reacts with sulfur in the air to make solid silver sulfide, the black material we call tarnish.
Ag (s) + H2S (g) + O2 (g) ® Ag2S (s) + H2O

23. #4. Balanced Chemical Equations
Atoms can’t be created or destroyed in an ordinary reaction:
All the atoms we start with we must end up with (meaning: balanced!)
A balanced equation has the same number of each element on both sides of the equation.

24. Rules for balancing:
Assemble the correct formulas for all the reactants and products, using “+” and “→”
Count the number of atoms of each type appearing on both sides
Balance the elements one at a time by adding coefficients (the numbers in front) where you need more - save balancing the H and O until LAST!
(hint: I prefer to save O until the very last)
Double-Check to make sure it is balanced.

25. Practice Balancing Examples
_AgNO3 + _Cu ® _Cu(NO3)2 + _Ag
_Mg + _N2 ® _Mg3N2
_P + _O2 ® _P4O10
_Na + _H2O ® _H2 + _NaOH
_CH4 + _O2 ® _CO2 + _H2O 2 2 3 4 5 2 2 2 2 2

26. Never
Never change a subscript to balance an equation (You can only change coefficients)
If you change the subscript (formula) you are describing a different chemical.
H2O is a different compound than H2O2
Never put a coefficient in the middle of a formula; they must go only in the front
2NaCl is okay, but Na2Cl is not.

27. Balancing Equations 2 2 ___ H2(g) + ___ O2(g) ---> ___ H2O(l)
What Happened to the Other Oxygen Atom?
This equation is not balanced!
Two hydrogen atoms from a hydrogen molecule (H2) combines with one of the oxygen atoms from an oxygen molecule (O2) to form H2O. Then, the remaining oxygen atom combines with two more hydrogen atoms (from another H2 molecule) to make a second H2O molecule.

42. Translate Equation ___ Al(s) + ___ Br2(l) →___ AlBr3(s)
Aluminum metal reacts with liquid bromine to form solid aluminum bromide
___ Al(s) 2
+ ___ Br2(l) 3
→___ AlBr3(s) 2